1. Field of the Invention
The present invention relates to a network access node of a digital communication system for the two-way transmission of message signals.
2. Description of the Prior Art
The term `network access node` refers to the unit of a digital communication system in which the message signals coming, for example, from a switching center, are processed for transmission to the individual subscriber terminals, taking into consideration the available transmission network. In this process, lines are frequently present on the side of the network access node facing the subscriber terminals in which several lines of a switching center are combined. If fewer channels are required in the front-end equipment than are available to it from the switching center, channels in the switching center are nevertheless assigned to the front-end equipment and are of no use to any subscriber.
An optical communication system is known from European Patent 0 318 331 A1 in which communication between the network node interface and subscribers is carried out by means of a TDM multiplex procedure in the downstream direction and a TDMA multiplex procedure in the upstream direction. In addition, the assignment of the lines of, for example, switching centers and front-end devices, are switchable by means of active optical couplers, which are located between the network node interface and the front-end devices.
However, couplers of this type permit only a switchover between lines of, for example, switching centers and a limited number of front-end devices if the number of optically-active couplers is to be restricted to a commercially-acceptable level.